Shaft induced electrical current is experienced in electrical motors, and is commonly experienced with three-phase motors driven by variable speed drives.
Variable speed drives utilize pulse width modulation technology to vary the speed of AC motors, thereby allowing use of less-expensive AC motors in applications where more expensive DC motors had been used previously. A drawback to the use of AC motors with variable speed drives is that higher common mode voltage (CMV) is generated by the variable speed drive that increases shaft induced currents.
Voltage on the motor shaft induces current flow through the shaft bearings to the motor frame and then to ground. While the motor is running, the bearings become more resistive to current flow, causing a buildup of charge on the shaft surfaces. Over a short period of time, the CMV causes electrical charges to build to a high level. As the electrical charges pass the threshold level of the least electrically resistant path, usually through the ball bearings on the shaft, an instantaneous burst or discharge of electrical energy passes through the bearing. This discharge causes electric discharge machining (EDM), which can damage the surfaces of the bearing races and the balls in the bearing. The electrical energy burst creates fusion craters, and particulate from the crater formation remains inside the sealed bearing. Both the fusion crater and the particulate material in the bearing act to disturb the free flow rotation of the bearing, which can lead to physical damage and premature bearing failure.
A number of mitigation technologies have been used in attempts to overcome this problem. Known attempts include using conductive bearing grease, insulating the bearings and using copper/phosphorus brushes and a Faraday shield. A common, somewhat cost-effective solution that has been used is to ground the shaft using spring-loaded copper brushes that provide a continuous flow of current to ground. Copper brushes, however, wear out quite rapidly and require frequent, periodic service and replacement. Additionally, oxide build-up on the shaft and other barriers between the brushes and the shaft reduce the current flow and cause a burst of electrical energy across the brush and shaft. Spring-loaded brushes also tend to vibrate due to alternating frictional stick/slip relationships between the brush and the shaft surface. Vibration of the brushes, from whatever cause, can result in undesirable sparking.
U.S. Patent Application Publication No. 2004/0233592, entitled “Grounding Brush For Mitigating Electrical Current On Motor Shafts,” assigned to Illinois Tool Works, Inc., (the “'592 publication”) discloses a grounding brush for mitigating static electric charge on a motor shaft. The grounding brush includes a plurality of filaments secured to an annular frame around the shaft, with the tips of the filaments disposed in a channel defined by the frame. The '592 publication overcomes many of the drawbacks of previous attempts to mitigate shaft electrical current.
The system disclosed in the '592 publication includes numerous individual filaments. When the system is mounted onto a motor shaft, proper alignment of the ring to the motor shaft depends only on the fiber filaments supporting the ring evenly around the shaft. With the exception of the fiber filaments, there is nothing to ensure that the filaments are properly aligned with respect to the motor shaft. In short, the filaments may be misaligned with respect to the motor shaft, thereby decreasing charge mitigation.
Thus, a need exists for an efficient grounding system that may be used effectively for a prolonged period of time, requiring minimal service or replacement.